Answer: I hope this will be the right answer (just joking)
Explanation: The major causes of difference between aquatic ecosystems:
First of all, if the aquatic ecosystem is a freshwater ecosystem or a wetland, (these are the least common on earth, only covering approx. 0.8% of the planet) the amount of saltiness in the water is way less or nonexistent (non-saline), than marine ecosystems and ocean ecosystems. So organisms living in freshwater ecosystems are not likely to survive the surroundings of a salty water ecosystem.
Secondly, we have the lotic and lentic water ecosystems: the first one lotic, is due when water is likely to flow rapidly, in contrast when the waters are more likely to be standing and not flowing, these are called lentic.
Finally, oceans normally have a bigger pressure than other water ecosystems, I think this would be another difference probably.
That would be the explanation about the differences among aquatic ecosystems, hope it helped!